Ocean acidification facts

The rate at which our oceans absorb carbon dioxide is currently 100 times faster than it has been for millions of years.

22 million tonnes of carbon dioxide are released by human industrial activity each day, and a quarter of it is absorbed into the ocean. The annual carbon dioxide emissions are thought to be around six billion metric tonnes.

Cold water absorbs larger quantities of carbon dioxide than warm water, and some areas of the polar seas are already so acidic that the water slowly dissolves the shells of molluscs and other shell-forming organisms.

Ocean acidification fact file

Introduction

Ocean acidification is known by many scientists as the ‘other’ carbon problem. The increasing amount of carbon dioxide emissions on Earth have not only resulted in a global temperature rise, but have also caused a change in the pH of the oceans, which are becoming more acidic. Since the industrial revolution, the amount of carbon dioxide created by the combustion of fossil fuels has risen exponentially, increasing the amount of this greenhouse gas in the atmosphere. It is thought that the oceans are 30 percent more acidic today than before the industrial revolution.

The balance of the Earth’s carbon cycle has been disrupted by the excessive carbon dioxide emissions created by humans. A certain amount of this greenhouse gas entering the ocean is required to allow marine vegetation to photosynthesise, although excess levels can be extremely damaging to the ecosystem.

The ocean acts as a ‘carbon sink’, absorbing over a quarter of the carbon dioxide released by industrial and agricultural activity. Forests are another example of a carbon sink, and they also remove vast quantities of carbon dioxide from the atmosphere. The amount of carbon dioxide that forests are able to remove from the atmosphere has decreased due to deforestation, and will continue to decline as the world’s forests are removed for commercial logging and land is cleared for agriculture.

Changing chemistry

Acidity and alkalinity are measured on a pH (potential hydrogen) scale and solutions with a high pH from 7.1 to 14 are classed as alkalis, those with a low pH from 1 to 6.9 are acids and those with a pH of 7 are neutral. Over the past few decades, scientists have studied the pH level of the oceans and found that the vast amount of carbon dioxide that has been absorbed by these water bodies has dramatically changed their chemistry. When carbon dioxide is absorbed into the ocean, it reacts to form a weak acid known as carbonic acid. To date, the pH of the ocean has been reduced by 0.1 units and it has been predicted that it will decrease by a further 0.3 to 0.4 units by the end of the 21st century.

This lower pH level has facilitated reactions between the seawater and the carbonate ions which naturally occur in the ocean, creating bicarbonate. This reaction has decreased the amount of carbonate ions in the ocean which are used by many species in shell formation and by corals to form the hard calcium carbonate exoskeletons that, over time, create coral reefs.

Are certain areas affected more by ocean acidification?

Carbon dioxide is absorbed by cold water much quicker than by warm water, therefore the oceans around the polar regions, namely the Arctic and Antarctic, are experiencing much faster rates of acidification than tropical areas.

In areas where other stressors such as pollution, acid rain or nutrient runoff are already having a negative impact on the marine environment, the effects of ocean acidification are more severe. In California, some marine zones have such high levels of acidity that the water dissolves the shells of molluscs and other shell-building organisms.

Which species are affected by ocean acidification?

Chemical changes in the ocean are causing a whole host of ecological problems for marine species, and the effects are evident worldwide. The process is happening so quickly that many species are unable to adapt and evolve, putting them at risk of extinction. As all marine species are already having to adapt to increasing sea temperatures and other stressors, the addition of this other issue could be the tipping point in their survival, especially if the population size has already been reduced. Species with smaller population sizes are more at risk of than those with a healthy population size.

Coral reefs

It is thought that coral reefs could be the first victims of ocean acidification, with one reef being destroyed every other day. The rate of disappearance of coral reefs is thought to be over twice as fast as that of rainforests. Coral reefs are built by many individual coral polyps, which secrete a hard calcium carbonate exoskeleton known as a ‘corallite’ to protect their vulnerable, soft body. To create these corallite exoskeletons, corals require aragonite and calcite, which are naturally occurring carbonate minerals present in the ocean. The occurrence of these essential minerals has greatly declined, and the rate of calcification in corals has reduced by 15 percent since 1990 and will continue to decline by up to 60 percent, which will be fatal for most reefs. Many coral reefs are already beginning to crumble, and when the calcium carbonate exoskeletons dissolve, more carbon dioxide is released, adding even more of this damaging gas to the ocean. The red and green algae that live in the corallite exoskeletons and make up around 90 percent of the total mass of coral reefs can also dissolve in acidic conditions.

Having a weakened exoskeleton increases the vulnerability of coral reefs to storms and heavy wave action. The frequency of extreme weather events is increasing due to climate change, and corals are becoming weaker and less able to cope with stormy sea conditions, which could lead to extensive damage and local extinctions during bad weather.

The Earth’s atmosphere currently has a carbon dioxide concentration of 380 parts per million, which is predicted to increase to 450 parts per million by 2030. When the atmospheric carbon dioxide concentration reaches this level, it is predicted that corals will be completely unable to form their exoskeletons due to a lack of calcite and aragonite in the ocean.

Ocean acidification is already slowing the growth of corals, and when their growth eventually stops, the reefs will gradually erode away. If nothing is done to combat ocean acidification, it is possible that all coral reefs may have disappeared by 2050. Around a quarter of all described marine species, which equates to roughly 1,000,000 species, rely on coral to survive and may become extinct if ocean acidification continues. As well as marine life, around 500 million people worldwide depend on coral reefs for their livelihoods and as a food source.

Molluscs, echinoderms and crustaceans

Mollusc, echinoderm and crustacean species such as mussels, sea urchins and lobsters all have a protective shell, which can be dissolved in acidic conditions, leaving the individual vulnerable to predation and disease. The decrease in availability of the molecules required for shell formation means that the individual spends a longer amount of time attempting to find them and less time finding food and reproducing, which has detrimental impacts on the overall population. This is especially damaging for species that have already experienced a decline in population size due to other stressors. Ocean acidification is responsible for some species creating deformed shells, which may also be detrimental to their survival.

Phytoplankton and plankton

Phytoplankton provides 50 percent of the atmospheric oxygen on Earth, removes a large amount of carbon dioxide from the atmosphere,and is the basis of the marine food chain. It is thought that its capacity to produce oxygen has reduced by six percent due to global warming and the effects of ocean acidification could reduce this further.

Non-photosynthesising planktonic organisms such aspteropods form protective shells and use the same materials as molluscs, crustaceans and echinoderms to do so. These materials are becoming less abundant as the ocean’s acidity increases. Both phytoplankton and plankton are at the base of the marine food chain, and their disappearance could have severely detrimental effects on the entire ecosystem.

Fish and marine mammals

The direct effects of ocean acidification will have repercussions on the entire marine food chain. The species which are directly affected are mostly at the bottom of the food chain, and their disappearance will reduce the amount of available prey items for other species, such as large fish and marine mammals, including the basking shark and humpback whale. It is likely that the lack of food will cause the population size of these species to decline, as well as many others. Humans will be negatively affected by fish population declines, as millions of people rely on the fishing industry for food and employment.

What can be done?

The only way to prevent the effects of ocean acidification is to reduce the amount of carbon dioxide emitted through the burning of fossil fuels.

If atmospheric carbon dioxide levels can be stabilised at 450 parts per million, it will still mean that only 8 percent of coral reefs in tropical waters will be in suitable conditions to survive.

Designation of Marine Protected Areas (MPAs) - although designating an area as an MPA will not prevent the absorption of carbon dioxide, the chosen area would be protected from industry and development. The absence of other stressors may allow the population number of many marine species to be re-established and would facilitate the development of the adaptations required to survive at lower pH levels.

Sustainable fishing practice - allowing fish population numbers to increase will help to improve the population’s health and subsequently help the organisms to adapt to the changing environment.

Research and monitoring - the monitoring of the physical, biological and chemical aspects of ocean acidification may enable scientists to develop effective management strategies.

Addition of base carbonate or silicate minerals to the ocean - acidity may be neutralised by adding alkaline materials to the ocean, although this method may not be very cost effective and its side effects on the marine ecosystem are unknown.

Encouraging the growth of ‘blue carbon’ areas - coastal mangrove forests, saltmarshes and seagrass meadows should be conserved, as they are able to uptake carbon up to 100 times faster than terrestrial forests. If these areas are conserved, they can assist in the removal of atmospheric carbon dioxide and will store it for many years, reducing the amount that is absorbed into the sea.

What can I do?

Reduce your carbon footprint - reduced carbon dioxide emissions means less absorption by the ocean. If every person reduced their carbon footprint, this would slow down the rate of ocean acidification and make it easier for species to adapt to the changing environment.

Try using different methods of transport - walk, cycle, carshare or use public transport to reduce vehicular emissions.

Use household electricity more efficiently - use energy saving light bulbs, lower the temperature on your thermostat, insulate your home and wash clothes in cooler water.

Eat more sustainably - eat sustainably sourced fish and less red meat, as well as organic food which has not been exposed to carbon-and nitrogen-rich fertilisers. Eating locally sourced food will also help to reduce your carbon footprint, as its transportation will have emitted much less carbon dioxide than food flown from abroad.

Holiday closer to home - airplanes emit vast quantities of carbon dioxide, so flying less will help to reduce your carbon footprint.

Glossary

Crustaceans

Diverse group of animals with jointed limbs and a hard external skeleton, characterised by the possession of two pairs of antennae, one pair of mandibles (mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, woodlice and barnacles.

Echinoderms

A group of marine animals that usually have a spiny skin. This group includes starfish, sea urchins and sea cucumbers.

Exoskeleton

An external skeleton that supports and protects an animal’s body.

Molluscs

A diverse group of invertebrates, mainly marine, that have one or all of the following: a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.

Photosynthesis

Metabolic process characteristic of plants in which carbon dioxide is broken down, using energy from sunlight absorbed by the green pigment chlorophyll. Organic compounds are made and oxygen is given off as a by-product.

Phytoplankton

Aquatic plants, usually tiny, that drift passively with water movements.

Polyp

Typically sedentary soft-bodied component of Cnidaria, a group of simple aquatic animals including the sea anemones, corals and jellyfish. A polyp comprises a trunk that is fixed at the base, and a mouth that is placed at the opposite end of the trunk and is surrounded by tentacles.

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These featured pages on ocean acidification have been created with support from Bank of America Merrill Lynch to help raise awareness of the plight of species being affected as a result of global climate change.